Cholesterol is essential for the proper structure and fluidity of cellular membranes and for the function of membrane proteins. Cholesterol also serves as a precursor of steroids, oxysterols, and bile acids. These metabolites serve important functions in signal transduction and, in the case of bile acids, lipid solubilization. Elevated serum cholesterol levels are associated with increased risk for cardiovascular disease (CVD). A clear understanding of how cells regulate cholesterol synthesis will accelerate the development of novel preventive and therapeutic approaches to CVD. In humans, the supply of cholesterol in the body is controlled by several factors including dietary intake, synthesis, storage, and excretion. Key regulators of cholesterol synthesis and metabolism are the cytochrome P450 monooxygenases. The synthesis of cholesterol from lanosterol requires the P450 enzyme, Cyp51A1. Our lab recently identified the heme-binding protein PGRMC1 as a binding partner and positive regulator of Cyp51A1 and demonstrated that shRNA knockdown of PGRMC1 in human embryonic kidney (HEK) 293 cells disrupts cholesterol synthesis. PGRMC1 and Cyp51A1 are both highly expressed in the liver-an essential organ in cholesterol metabolism. Based on these findings, I hypothesize that PGRMC1 is a critical regulator of hepatic cholesterol synthesis and is necessary for maintaining systemic cholesterol homeostasis. My aims are to: (1) Determine the function of PGRMC1 in hepatic cholesterol synthesis and (2) Investigate the role of PGRMC1 in systemic sterol homeostasis. Together, these investigations will help elucidate the function of PGRMC1 in cholesterol homeostasis.